1. Field of the Invention
The present invention relates to a technique for preventing turbulence-induced accidents by irradiating a laser beam into the atmosphere and receiving the laser beam scattered in the atmosphere, and more specifically to a device that prevents turbulence-induced accidents using an airborne Doppler lidar that measures a wind velocity in a remote region at a distance of from about several hundreds of meters to ten odd kilometers on the basis of the Doppler effect.
2. Description of the Related Art
Turbulence has recently attracted attention as a main cause of aircraft accidents, and a Doppler lidar using a laser beam has been researched and developed as an airborne device that detects turbulence in advance (see, for example, H. Inokuchi, H. Tanaka, and T. Ando, “Development of an Onboard Doppler lidar for Flight Safety,” Journal of Aircraft, Vol. 46, No. 4, pp. 1411-1415, July-August 2009). Lidar is an abbreviation for “Light Detection And Ranging”, that is, a technique for detection that uses light. With this technique, an irradiated light beam is scattered by fine aerosol floating in the atmosphere, the scattered beam is received, and the frequency variation amount (wavelength variation amount) according to the Doppler effect is measured, whereby the wind velocity is measured. Accordingly, the method is called Doppler lidar. Airborne weather radars that have already found practical use have an effective range as wide as several hundreds of kilometers. In the usual mode, the observation screen is displayed at all times and the pilot can monitor the screen as necessary, thereby making it possible to take the appropriate measures in advance. However, since the weather radars use scattering of microwaves on water droplets contained in the atmosphere, they are not effective when the sky is clear. By contrast, the Doppler lidar is effective when the sky is clear, but the effective range is greatly limited by comparison with that of the weather radars, and therefore the avoidance time interval is short. Moreover, with this method, the pilot watches the display at all times which undesirably increases workload. Therefore, where the device automatically determines the impending danger or abnormality and a function of notifying the pilot is provided, the pilot can monitor the display only upon receiving the notification and therefore the load on the pilot can be reduced.
Even when the presence of a turbulent region in front in the direction of the aircraft flight is clearly established, in the case in which the turbulent region cannot be avoided due to aircraft characteristics or the abrupt avoidance maneuver can by itself cause danger, the turbulent region is not avoided and the accident is prevented by performing a flight such that minimizes fuselage shaking when the aircraft enters the turbulent region (see, for example, Masayuki Sato, Nobuhiro Yokoyama, and Atsushi Sato, “Gust Alleviation via Robust Model Predictive Control Using Prior Turbulence Information” Journal of the Japan Society for Aeronautical and Space Sciences, Vol. 57, No. 9, 2009). Thus, when a pilot determines that the turbulence cannot be avoided, since the characteristics of the Doppler lidar are suitable for displaying at the dashboard in the cockpit, automatic gust alleviation control can be performed by adding a function of switching to a mode suitable for autopilot input.
The inventors have previously filed a patent application relating to “Wind Terbulence Prediction System” (Japanese Patent Application Publication No. 2003-14845, published on Jan. 15, 2003; U.S. Pat. No. 6,751,532). The object of the invention disclosed in this patent publication is to provide a measurement system that can measure three-dimensional wind terbulence and can verify in advance whether a warning is reliable, instead of producing a surprising sudden warning such as in the conventional wind shear system. This system can detect wind terbulence in a form such that measures that need to be taken can be easily determined, exhibits limited aerodynamic and structural effects when mounted on an aircraft, and is capable of measurements and produces no positional error even at a velocity equal to or less than 20 to 30 m/s, at which a Pitot tube is incapable of measurements, and when the airflow direction differs greatly from the fuselage axis. In this wind terbulence prediction system, a laser wind velocity meter of a coherent configuration incorporating a heterodyne receiver is mounted on an aircraft, a laser beam is irradiated, while being scanned in a cone shape, and scattered light from a wind terbulence region forward of the flying aircraft is received, whereby the three-dimensional air flow velocity in a remote region is measured. With consideration for the effect produced by a vertical wind and a fore and aft wind, the measured three-dimensional air flow information is converted into a vertical wind alone and displayed in a simplified form in two dimensions, and wind terbulence is expressed by breakdown thereof into a turbulence intensity and an average wind. Further, when the measured wind velocity information is transmitted to the pilot, the time that elapses before the aircraft encounters the turbulence, rather than the distance, is used as a reference for displaying the terbulence position, and parts of the cylindrical optical system of the wind measuring lidar is cut away, thereby facilitating the mounting thereof.
However, the specific nature (characteristic) of this system is such that since the reception intensity of the laser scattered light detected by the Doppler lidar of this kind is inversely proportional to a second power of the distance between the device and the measurement region, the received signal intensity is generally high and the measurement accuracy increases in near-range measurements, but as the distance increases, the received signal intensity decreases with respect to the internal noise component, the ratio of incorrectly measured values gradually increases, and measurement reliability decreases. Conventional methods designed to improve the far-range measurement performance involve increasing the transmission output and expanding the reception area, but both these methods unavoidably lead to the increased size of the device or raise cost due to increased energy consumption. In particular, when a device is mounted on an aircraft, electric power for driving the device and a space that can be used for mounting are limited, and at a high altitude where passenger aircraft cruise, the amount of aerosol in the atmosphere decreases. As a result, the performance degradation cannot be avoided and the possibility of improving the measurement capability by a method increasing the transmission output is limited.